Here's a post I wrote where I summarized the various options. It's a couple years old at this point, but I think all the model names and whatnot are still applicable. I'd go with an Xbee 3 now unless you have a reason to use (or can only find) the S2C, but I think everything else still holds:
As I mentioned there, the two main pieces you'll need are mostly the Xbee module itself and a "board" to power and, for at least configuration, provide a data connection to it. Digi's Xbee Grove Dev Board is a full-featured choice for the latter but not super cheap--but unlike super-cheap options from random vendors, you'll know it works. Other boards can usually be made to work, and I only have one or two Grove boards and use cheaper ones to just power my devices when they're in use. If you only have one Xbee, you may just want to take the easy route. (You'll also need some things you probably already have, like a micro USB cable to connect things.)
For where to buy: DigiKey and Mouser are popular choices for where to buy these components if you're in the US. I've usually used DigiKey. eBay and Amazon probably have something, too, but the specific models may not be as easy to find.
